I don't think that there's any question in terms of whether or not the NS will use the B&A for IM. They'll be able to improve their economics and grow business by having the ability to DS and will also be able to speed up service vs using the B&M.
Perhaps it’s a question of when, not if, NS exploits the CSX trackage rights. NS must fund a significant trackage rights start up cost by 1) rehabbing the Albany Main between Delanson and Voorheesville, 2) paying CSX to install a new turnout onto the Selkirk branch presumably remotely controlled and wired into CSX CTC/PTC, 3) paying for clearance improvements over P&W and Worcester main to Harvard.
NS has had IM trackage rights over PAS from Mechanicville to Ayer since its inception in 2009 but for reasons unknown to me has never opted to hire crews to run 22K/23K over PAS.